Metasploit is described as 'Community Edition simplifies network discovery and vulnerability verification for specific exploits, increasing the effectiveness of vulnerability scanners such as Nexpose - for free. This helps prioritize remediation and eliminate false positives, providing true' and is a vulnerability scanner in the security & privacy category. There are more than 10 alternatives to Metasploit for a variety of platforms, including Linux, Windows, Mac, Web-based and Self-Hosted apps. The best Metasploit alternative is mimikatz, which is both free and Open Source. Other great apps like Metasploit are Exploit Pack, Sn1per Professional, Social-Engineer Toolkit and PhoneSploit Pro.
Nessus scans for vulnerabilities, Metasploit exploits upon the vulnerabilities. I guess the prices are similar though...